Method and apparatus for controlling power division in a travelling-wave antenna

ABSTRACT

This publication discloses a method and apparatus for feeding signal power to an antenna array comprising several antenna elements ( 1 - 4 ). According to the method, the signal power is fed to the antenna elements ( 1 - 4 ) over two transmission lines (a-b) and (c-d), in which case the divisions (e) of one of the transmission lines (a-b) are shorter than the nominal wavelength and those (f) of the other (c-d) are longer than the nominal wavelength by the same amount, the signal s being fed is divided into two parts, one of which is led to one of the said two transmission lines (a) and the other to the opposite end (d) of the other line, in which case the signals travel in the lines in opposite directions, and the signals are summed ( 5 ) before the antenna elements ( 1 - 5 ).

The present invention relates to a method, according to the preamble ofclaim 1, for controlling power division in a travelling-wave antenna.

The invention also relates to an apparatus for controlling powerdivision in a travelling-wave antenna.

The invention is intended to permit a desired form of amplitudedistribution when feeding the transmission line of a travelling-waveantenna alternately from each end, in order to turn the lobe of theantenna to two different positions.

THE PRIOR ART AND ITS DRAWBACKS

In a travelling-wave antenna, power is distributed to the radiatingantenna elements along a serial line (FIG. 1), in which the phase of thesignal going to the antenna elements (1, 2, 3, and 4) from one end (a)depends on the lengths of the parts of the transmission line and theamplitude on the division ratios of the connection points. If thetransmission-line parts are of a length that is exactly the wavelength,the phase difference between the antenna elements will be zero, and thelobe of the antenna will be aligned perpendicular to the plane of theantenna elements.

If the length of the parts of the transmission line differs from thewavelength, there will be an equal phase difference between all theantenna elements, and the lobe of the antenna will turn away from theperpendicular position. If the transmission line is fed from the otherend (b), the said phase difference will have the opposite sign, and thelobe will turn to the other side of the perpendicular position. Thus,the lobe of the antenna array can be turned to two positions, by using asuitable switch to change the direction of travel of the signal.

If the transmission line is fed from only one end a, the division ratiosof the line can be dimensioned to give the antenna elements the desiredamplitude distribution. If a transmission line designed in this way isfed from the other end b, the division ratios will no longer be theprevious ones, while the characteristics of the lobe created will not bethose desired. If the division ratios are to remain identical when theline is fed from both ends, a constant division ratio must be applied inthe divisions of all the antenna elements, 1, 2, 3, and 4, which willresult in the exponential diminution of the amplitude distribution. Theside-lobe level of the antenna array will then increase.

The invention is intended to eliminate the defects of the state of theart disclosed above and for this purpose create an entirely new type ofmethod and apparatus for controlling the power division of atravelling-wave antenna. By means of the solution according to theinvention, the lobe of a travelling-wave antenna can be turned to twodirections using a transmission line fed from different ends, so thatthe drawbacks described above are mostly avoided.

The invention is based on using two parallel transmission lines, thedivisions of one of which being shorter than the nominal wavelength andthose of the other being longer than the nominal wavelength by the sameamount, dividing the signal s being fed into two parts, one of which isdirected to one of the two said transmission lines a and the other isdirected to the opposite end d of the other line, so that the signalstravel in opposite directions in the lines. In that case, the signalswill be in the same phase at the corresponding divisions in thetransmission lines. The same-phase signals diverging from pointscorresponding to the position of the divisions of the transmission linesare summed and directed to the antenna elements 1, 2, 3, and 4.

More specifically, the method according to the invention ischaracterized by what is stated in the characterizing portion of claim1.

The apparatus according to the invention is, in turn, characterized bywhat is stated in the characterizing portion of claim 3.

Considerable advantages are gained with the aid of the invention.

The amplitude distribution is identical in both positions of the antennalobe.

Altering the division ratio allows the amplitude distribution to beoptimized, in order to achieve the desired characteristics of the lobe.

In the following, the invention is examined with the aid of examples andwith reference to the accompanying drawings.

FIG. 1 shows a schematic diagram of the feed line of a travelling-waveantenna according to the prior art.

FIG. 2 shows a schematic diagram of a dual-travel-wave feed lineaccording to the invention.

FIG. 3 shows graphically the amplitude distribution of the transmissionlines, when using a constant division.

Thus, in the solution according to the invention, operation is asfollows:

1.) In the solution, two parallel transmission lines are used, thedivisions e of one being shorter than the nominal wavelength and thedivisions f of the other being longer by the same amount than thenominal wavelength (FIG. 2). Thus, when the signal travels in the lines,in the same direction, the phase-differences arising in the divisionswill be opposite.

The power-division ratios of the transmission lines are typically atleast approximately identical.

2.) The signal s being fed is divided into two parts, one of which isdirected to the end a of the transmission line a-b and the other to theopposite end d of the other line c-d, so that the signals travel inopposite directions in the lines. In that case, the signals will be inthe same phase at the corresponding divisions.

3.) The same-phase signals diverging from the corresponding points atthe locations of the divisions of the transmission lines are summed in asumming unit 5 and directed to the antenna elements 1, 2, 3, and 4.Thus, when using constant power-division ratios, the amplitudedistribution of both lines will be identically exponentially diminishing(FIG. 3). Due to the summing that takes place in the summing amplifier 5preceding the antenna elements 1, 2, 3, and 4, the final amplitudedistribution is symmetrical. If it is wished to reduce the side-lobelevel of the antenna array, the signals from the divisions 1, 4 at theends of the transmission line are directed to the centre of the antennaarray and from the divisions 2, 3 in the centre to the outer ends of thearray (FIG. 3), as the amplitude of the signal is greatest at thedivisions at the ends. The final amplitude distribution can becontrolled by altering the division ratio of the divisions.

4.) If it is wished to alter the direction of the lobe, the transmissionlines are altered to feed from the point b and c, in which case thesignals will travel in a different direction, though the mutualphase-differences will remain the same at the points 1, 2, 3, and 4. Inpractice, this change in feed takes place with the aid of a switch,which is not shown. Because the amplitude distribution is symmetrical,it will remain identical in both positions of the antenna lobe, when thedirection of travel of the signal is altered. If it is wished to reducethe side-lobe level of the antenna array, the signals from the divisions1, 4 at the ends of the transmission line are directed to the centre ofthe antenna array and from the divisions in the centre to the outer endsof the array, because the amplitude of the signal is greatest at thedivisions at the ends. The final amplitude division can be controlled byaltering the ratio of the divisions.

1. A method for feeding signal power to an antenna array comprisingseveral antenna elements (1-4), which method is characterized in thattwo parallel transmission lines (a-b) and (c-d) are used, in which casethe divisions (e) of one (a-b) of the transmission lines are shorterthan the nominal wavelength and those (f) of the other (c-d) are longerthan the nominal wavelength by the same amount, the signal s being fedis divided into two parts, one of which is directed to one of the saidtwo transmission lines (a, (a-b)), and the other to the opposite end ofthe other line (d, (c-d)), so that the signals travel in the lines inopposite directions, and the signals are summed (5) before the antennaelements (1-5).
 2. A method according to claim 1, characterized in thatthe direction of the lobe created by the antenna elements (1, 2, 3, 4)is changed by feeding the transmission lines ((a-b), (c-d)) from theiropposite ends (b, c), in which case the signals travel in a differentdirection in the transmission lines, but the mutual phase-differencesremain essentially unchanged.
 3. An apparatus for feeding signal powerto an antenna array comprising several antenna elements (1-4), whichapparatus includes several antenna elements (1-4), and two transmissionlines (a-b) and (c-d) for feeding signal power to the antenna elements,characterized by the divisions (e) of one of the transmission lines(a-b) being shorter than the nominal wavelength and those (f) of theother (c-d) are longer than the nominal wavelength by the same amount,means for dividing the signal s being fed into two parts, one of whichis directed to one of the said two transmission lines (a, (a-b)) and theother is directed to the opposite end of the other line (d, (c-d)), sothat the signals travel in the lines in opposite directions, and summingelements (5), by means of which the signals coming from the differenttransmission lines can be summed before the antenna elements (1-5).